Single-walled carbon nanotubes (SWCNTs) have been attracting researchers' attention for potential applications as field emission transistors for electronic devices, computers and thin film transistor backplanes. The mixed nature of SWCNTs growth, however, has impeded their implementation. One reason if the unavailability of high purity, single chirality SWCNTs. Current methods of purifying SWCNTs rely on optical spectroscopic screening techniques that have proven unable to accurately establish CNT purity. Devices using semiconducting SWCNTs deemed ‘pure’ by optical screening methods ubiquitously show linear current-to-bias (“I-V”) responses, violating semiconductor characteristics of metal/semiconductor Schottky contacts and illustrating the presence of metal impurities and metallic SWCNTs.
Semiconducting single-walled carbon nanotubes have demonstrated the ability to be used in place of high performance silicon transistors for applications in microprocessor and radio frequency devices. Semiconducting single-walled carbon nanotube (SWCNT) thin film transistors (TFTs) also exhibit promise for large size display backplanes. The majority of SWCNT TFTs were bottom gated with SiO2 or A12O3. The device performance on these bottom gated SWCNT FETs were unstable and degraded after certain time, requiring polymer encapsulation or inorganic thin film passivation. In contrast, top-gated SWCNT TFTs are stable and promising for real applications. A few top gated SWCNT TFTs have been reported using dielectric materials such as HfO2, A12O3, ZrO2, and Y2O3 deposited using electron beam evaporation or atomic layer deposition. The dielectric materials of these successful devices were all deposited at low temperature (<150° C.).
Recently, the SiNx commonly used for amorphous silicon TFTs was adapted to be used as dielectrics for SWCNTs TFTs. Silicon nitride gate dielectrics for top-gated carbon nanotube field effect transistors showing p-type characteristics were obtained using plasma enhanced chemical vapor deposition (PECVD) at 225° C. Both n-type and p-type characteristics of SWCNTs transistors with SiNx passivation films or top-gated insulators have been observed based on different deposition temperatures using catalytic chemical vapor deposition. At deposition temperatures higher than 330° C., SWCNTs were destroyed. At a deposition temperature around 270° C., the fabricated transistors were converted from p-type to n-type characteristics. This was interpreted as due to the removal of the adsorbed oxygen. At deposition temperatures between 60° C. and 120° C., the carbon nanotube transistors retained their original p-type properties. Stable n-type SWCNTs TFTs have also been obtained by annealing the devices with a Si3N4 layer deposited in a plasma-enhanced chemical vapor deposition system at 110° C. in nitrogen atmosphere at 200° C. for one hour, or by using PECVD directly deposited Si3N4 as dielectrics. More recently, SiO2 bottom gated n-type SWCNTs TFTs having SiNx passivation deposited at 150° C. using PECVD were reported. No damage was induced using PECVD at 150° C., and the obtained n-type characteristics were attributed to the doping of SWCNT by SiNx K (Si≡N+) centers, which sufficiently thinned the Schottky Barrier (SB) to the conduction band to allow for efficient electron tunneling from the contacts into nanotubes. The effects of metal/SWCNT contacts were attributed to the wettability of metals to carbon nanotubes.
Schottky barriers occur when the semiconductors contact with the metals. Evidence of Schottky barriers was observed as inflection points in output characteristics of semiconducting carbon nanotube field effect transistors. The linear conductances at low drain bias in the output characteristics of SWCNT FETs with Schottky barriers have been attributed to the tunneling effects, and the on-conductances (4e2/h) are used to determine Schottky barriers of SWCNT TFTs. Thus SWCNT TFTs have been considered Schottky Barrier transistors for the modulation of the contact resistances, with the exclusive focus on transmission through the barrier by thinning the barrier and increasing the tunneling. These theoretical explanations were based on back-gated SiO2 dielectrics that are purer and more defect free than silicon nitride dielectrics.
Although advances in SWCNT TFT performance has made some progress, devices with performances rivaling those of amorphous silicon based devices has been hampered by the quality of the available SWCNTs.